As is known in the art, it is frequently desired to transition from one type of microwave transmission line to a different type of microwave transmission line, for example between coplanar waveguide transmission line to microstrip transmission line. As is also known in the art, one type of a transistor device used in amplifiers includes a control electrode (e.g., a gate electrode in a field effect transistor (FET) device or a base electrode in a bipolar device) for controlling carrier flow between a pair of electrodes (e.g., source and drain electrodes in a FET and collector and emitter for a bipolar transistor).
As is known in the art, one type of transistor arrangement includes a transistor coupled between a microwave transmission line input section and a microwave transmission line output section. Thus, there is a transition between the input section and the transistor and another transition between the transistor and the output section.
One type of microwave transmission line is a microstrip transmission line having a strip conductor on one surface of a substrate and a ground plane conductor on an opposite surface of the structure under the strip transmission line. With such a microstrip transmission line the electric field of the microwave energy propagating through the transmission line is within the substrate between, and perpendicular, to the strip conductor and the ground plane conductor.
Another type of microwave transmission line is a coplanar waveguide (CPW) transmission line where the strip conductor and ground plane conductor are on the same surface of the substrate (i.e., the strip conductor and ground plane conductor are coplanar). More particularly, the strip conductor provides a center conductor for the waveguide and the ground plane conductor has two sections; one on either side of the strip conductor. With such a coplanar waveguide transmission line the electric field of the microwave energy propagating through the transmission line is adjacent to the surface of the substrate between the strip conductor and the ground plane conductor.
As is also known in the art, with one type of transistor, the control electrode includes an electrical conductor disposed on one surface, for example the upper surface, of a semiconductor substrate, such conductor has a conductive pad terminating in a plurality of parallel conductive fingers (sometimes called gate fingers). One of the aforesaid pair of electrodes (e.g. drain) includes an electrical conductor disposed on the upper surface of a semiconductor substrate having a conductive pad terminating in a plurality of conductive fingers disposed between selected pairs of the inner control electrode (e.g. gate) fingers. The other one of the pair of electrodes (e.g. source) has a plurality of electrically conductive pads disposed on the upper surface of the substrate. Each one of the control electrode (e.g. gate) fingers is disposed between a corresponding one of the first electrode (e.g. drain) fingers and a corresponding one of the second electrode (e.g. source) conductive pads. A plurality of air bridge conductors is disposed over the surface of the structure electrically interconnecting the plurality of second electrode conductive pads. The second electrode conductive pads adjacent the outer control electrode fingers are connected to a ground plane conductor on the bottom surface of the substrate with vias passing vertically through the substrate to form a FET device as shown in FIGS. 1A-1D. Thus, the structure has an input section comprising an input microstrip transmission line feeding the control (here) gate electrode of the transistor, and an output microstrip transmission line one of the aforesaid pair of electrodes, here drain electrode of the transistor and the second one of the aforesaid electrode, here source electrode, being grounded. Thus, the transistor is arranged as an amplifier.
As is also known in the art, a coplanar waveguide (CPW) transistor structure is often times desired (for example, to reduce source inductance and achieve higher gain). Here, as shown in FIGS. 2A, 2B, 2C and 2D, the input microwave transmission line and the output transmission line are coplanar waveguide (CPW) transmission lines. Thus, the input section has a strip conductor disposed as the center conductor between a pair of coplanar ground planes. Likewise, the output section has a strip conductor disposed as the center conductor between the pair of coplanar ground planes. More particularly, the pair of ground planes extend across the structure as shown and provides the ground plane conductor for the pads of the gate and drain electrodes. Thus, the electric field (indicated by the arrows) passing through the CPW input section is coupled to the gate electrode and is along the surface of the substrate. Likewise, the electric field passing through to the CPW output section is coupled from the drain pad and along the surface of the substrate. Thus, here the transistor is a CPW transistor because microwave energy is fed to it and coupled from it with CPW transmission lines. A disadvantage of CPW input/output impedance matching circuits (not shown) coupled to the CPW input and output sections is that their implementation requires a greater surface area than that required with the microstrip configuration of FIGS. 1A-1D).
As reported in an article entitled “Monolithic GaAs W-Band Pseudomorphic MODFET Amplifiers” by Sequeira et al., IEEE GaAs IC Symposium, 1990, pages 161-164, a microstrip-CPW structure is shown wherein the input and output sections are microstrip and the transistor is CPW. Here, the ground plane of the microstrip conductors is on the same surface as the ground plane conductor of the gate and drain pads of the coplanar waveguide transistor. More particularly, the strip conductors for the input section and output microstrip section along with the strip conductors for the input/output microstrip impedance matching networks are on the bottom surface of the substrate and the ground plane conductors of the microstrip input/output/impedance matching circuits are on the same (upper) surface as the gate and drain pads of the CPW transistor. These bottom strip conductors of the input and output sections then connect to the gate and drain electrode center conductors of the CPW transistor with vertically extending vias. Such an arrangement is difficult to package.